A display device such as a liquid crystal display generally comprise a set of active picture elements or pixels arranged in an orthogonal matrix of rows and columns. Each pixel is individually illuminated by applying a voltage corresponding to the brightness to be displayed. The applied voltages are obtained from pixel driver circuits that convert received display information into drive voltages. Each driver circuit is individually associated with one column of the pixel matrix. The rows of the matrix are sequentially scanned and individual pixels of the row being scanned are illuminated to prescribed gray scale levels in response to the signals from the drive circuits.
The information to be displayed at a pixel is generally a digital coded signal or an analog video signal which is transformed into a digital code representing a discrete brightness level. A brightness level code is provided for each column of pixels in the display device. Color television and other high quality liquid crystal displays typically include 1440 columns. There may be 240 to 1152 rows depending on the display size and the rows are selected sequentially. Upon selection of a row, the 1440 data lines supply the pixels of the selected row with gray scale level signals. Consequently, 1440 data lines are required to provide the display columns with brightness level digital codes for each selected row and the data line drive circuits must operate at high rate to produce a satisfactory display.
U.S. Pat. No. 4,742,346 issued to Glynn Gillette et al., on May 3, 1988, discloses a system for applying gray scale coded brightness signals to the pixels of a display device in which data stored in shift register segments are transferred to counters through multiplex circuitry. Each counter controls a column transfer gate which applies an analog signal such as a ramp signal to charge the selected pixel while the transfer gate is enabled. The transfer gates are turned on at the selection of each display row. The counters are decremented at a fixed rate and each transfer gate is turned off when the stored count for the column is decremented to zero. While the counter arrangement simplifies transfer of brightness levels to the display pixels, the counting device must support a high counting rate. Liquid crystal displays, however, generally use thin film devices (e.g., field effect transistors) deposited on the display substrate. Thin film field effect transistors and therefore it is difficult to obtain a high count rate in a counter that uses such transistors. As higher quality displays are required, it is necessary to increase the count rate and the counter controlled transfer gate arrangement is more difficult to implement. It is desirable to provide a high rate timing control using limited speed devices such as thin film transistors.